The present invention relates to repair of screens used in high temperature applications, and more particularly, to a method for repair of stainless steel screens used in gas pressurized fuel systems.
The principal on which all spacecraft surface tension acquisition fuel systems are based is that a wetted screen surface offers negligible resistance to fluid flow but resists gas flow up to a defined pressure difference known as bubble point. Bubble point is a function of screen opening size and in some applications bubble points equivalent to pressures of 18 inch of water are required. If this bubble point or pressure is reduced due to small damages areas or defects, these areas have to be repaired to prevent excessive pressurant gas flow and resultant entrained gas bubbles in the spacecraft fuel and reduction in performance. Excess screen surface area is normally available so that blockage of small areas with repairs will not affect the propellant flow requirements.
In the Peacekeeper Missile, stainless steel screens are used in the fuel surface tension acquisition system. These screens are fabricated from fine low carbon stainless steel wires, such as 304L. They are woven to a fine mesh, either 200.times.1400 mesh, having warp and shute diameters of 0.0028 and 0.0016 inches, respectively, or 325.times.2300 mesh, having warp and shute diameters of 0.001 and 0.0015 inches, respectively.
Repair of these fine meshes can be difficult. If the fuel tank system is not exposed to elevated temperatures either in the manufacturing cycle or in service, then repairs such as teflon or soldering can be used. But these materials have a use temperature of about 800.degree. F. For components exposed to temperatures greater than 800.degree. F. either in the manufacturing cycle or in operation these materials are not satisfactory. The Peacekeeper tanks are exposed to 1000.degree. F. in the manufacturing cycle.
Plasma spray guns are available which utilize an electric arc contained within a water-cooled jacket. An inert gas, such as argon, is passed through the arc and is excited to temperatures of up to 30,000.degree. F. The plasma of ionized gas issuing from the torch resembles an open oxyacetylene flame in shape and appearance. In use, a powdered material is controlledly fed into the plasma in the gun and is carried thereby to the base material to be coated. There are two major problems with application of this process to the repair of small holes in the abovesaid finely meshed screens. First, the heat generated by the plasma flame can easily destroy such screen material. Second, the plasma flame spreads out as it issues forth from the gun nozzle. Hence, pin point repair using such device is made difficult.
It is, therefore, an object of the present invention to provide a method for high temperature repair of finely meshed stainless steel screens which can withstand temperatures on the order of 1000.degree. F.
It is another object of the present invention to provide a method for precision repair of finely meshed stainless steel screen employing a plasma spray process.